1. Field of the Invention
The invention relates to an electric power steering system.
2. Description of Related Art
Conventionally, there is an electric power steering system (hereinafter, referred to as “EPS”) that assists a driver's steering operation by supplying power generated by an electric motor to a steering mechanism of a vehicle. For example, a controller of an EPS described in Japanese Patent No. 4453012 controls an electric motor based on a steering torque, a steering angle, and a wheel steered angle acquired by various sensors.
The controller has first and second normative models (models in which the purpose of the control is formulated). The first normative model defines a relationship between a steering angle and a target steering torque. The second normative model defines a relationship between a steering torque and a target steered angle. The controller executes proportional-integral-derivative control (PID control) that is a kind of feedback control based on the target steering torque and the target steered angle that are respectively determined by the first and second normative models.
The controller computes a deviation of an actual steering torque from the target steering torque determined by the first normative model and a deviation of an actual steered angle from the target steered angle determined by the second normative model. Then, the controller controls the electric motor so as to eliminate the deviations. The controller controls the electric motor to cause the actual steering torque and the actual steered angle to follow the target steering torque and the target steered angle, respectively.
The second normative model defines the target steered angle that is an ideal steered angle corresponding to the steering torque. Through the feedback control of the steered angle executed based on the second normative model, it is possible to reliably obtain a rigid steering feel, but a driver's sense of unity with a vehicle may be insufficient depending on driving conditions. For example, when a steering wheel is turned largely, the driver senses a lateral acceleration corresponding to the operation amount of the steering wheel, but the response (steering reactive force) that the driver senses at his/her hands from the steering wheel does not change. This is because the steering reactive force corresponds only to the target steered angle defined by the second normative model. As a result, the driver may feel uncomfortable due to an imbalance between the magnitude of the lateral acceleration actually sensed by the driver's body and the response that the driver senses at his/her hands from the steering wheel.